If you are used to using your clothing to sleep (in CO winter?!) what sort of clothing are you bringing in what sort of conditions? The average human needs 2x the amount of insulate to sleep comfortably as they need when standing around. With such warm clothing you should make sure your sleep system can incorporate your clothing. You might want to make a DIY quilt which would provide enough room so your clothing's loft doesn't get collapsed.
The sub kilo is a pretty good deal at $130. Two things I would note (1) it's a pretty narrow bag with 57" shoulder girth. Works fine for several of my friends, but I wouldn't use because I find anything less than around 61" too confining. (2) Based on my friends experiences, I would guess that REI is using the "keep you safe" rating value to have the subkilo at 20F. Unless you run hot, I don't think you will be really "comfortable" down to 20F in the sub kilo wearing just a base layer. Extra clothing should be fine. Typically the cheapest sleeping bags will be to find a good quality used down bag.
I don't have any personal experience with the Utopia 2. There was a brief discussion in a BPL thread about using the utopia 2 in snow. Provided you aren't trying to keep out crawling bugs or standing water (e.g. winter), the Utopia 2 in specific, a floorless shelter in general with a ground cloth (I recommend polycro) works great. I generally prefer pyramid tarps for the winter.